The present invention generally concerns ion-exchange resins and, in particular, a process for preparing improved ion-exchange resins by partially functionalizing non-uniform size copolymer beads with ion-exchange functional groups.
Ion-exchange resins are used widely for purification of various substances. Such resins are used in water treatment and purification, food preparation, pharmaceutical manufacturing, chemical processing, metal extraction, and so on, as is generally discussed by Wheaton et al, in, "Ion Exchange", 11 Kirk-Othmer's Ency. Chem. Tech. pp. 871-899 (2nd Ed. 1966).
In general, ion-exchange resins comprise a plurality of polymeric matrices typically in the form of spheroidal beads, or pearls, which are generally formed by suspension polymerization methods well-known to the art. The polymeric matrix has a plurality of attached functional groups which are capable of retaining ions, or molecules, of a chemical species when in contact with a liquid containing such species.
The nature of a suspension polymerization process is such that the resulting copolymer beads exhibit differing particle sizes, i.e., the beads collectively fit into a distribution of particle sizes. This distribution is retained, and in most instances widened, when the copolymer beads are substantially functionalized with ion-exchange groups. Many ion-exchange resins in commercial use today have broad particle size distributions.
However, of particular interest to industry are ion-exchange resins of relatively uniform particle size. These resins are desired due to their generally superior operating performance in commercial ion-exchange processes. For example, small resin particles have shorter diffusion paths into the particle for the species to be retained, which results in improved exchange kinetics when compared to larger resin particles. However, small resin particles generally tend to increase the pressure drop across a resin bed, which limits the amount of liquids that can be processed. Resin beads of fairly uniform size allow for use of generally smaller particles with their desirable exchange kinetics, since there are reduced, or minimal, amounts of very fine resin particles that would otherwise contribute toward unacceptably high pressure drops. Uniform size ion-exchange resins also generally have superior exchange kinetics in comparison to non-uniform resins. The superior exchange kinetics are due to a relatively short uniform diffusion path length for the chemical species being retained therein. The term "uniform diffusion path length" generally refers to the bead radius being essentially the same for each resin particle.
Chromatographic separations of various substances can be accomplished using ion-exchange resins as the stationary phase. Such processes use anion- or cation-exchange resins to separate, for example, mixtures of organic compounds. Of particular commercial importance is the separation of fructose from glucose and oligosaccharides in the production of high fructose-containing syrups. In this process, liquid mixtures of glucose and fructose are passed through one or more columns of a strong acid cation-exchange resin, most typically in the calcium form. The passage of the fructose through the column is retarded relative to that of the glucose, so there can be obtained separate product streams containing high proportions of fructose and glucose, respectively.
Uniform size resin particles are also advantageous for chromatographic separations to obtain sharp separations and maintain uniform flow through a chromatography column. These advantages are discussed in detail in U.S. Pat. Nos. 3,928,193 and 4,543,261, the relevant disclosures of which are incorporated herein by reference.
Industry has developed methods to produce ion-exchange resins of substantially uniform size. Such methods are directed toward preparing relatively uniform size monomer droplets which are thereafter polymerized to obtain copolymer beads having a narrow particle size distribution. Upon functionalization, the resulting ion-exchange resin exhibits a similar, narrow particle size distribution. Examples of such methods are found in U.S. Pat. Nos. 4,444,961; 4,427,794; and 4,487,898. These methods are generally quite complex and require large amounts of capital to provide the necessary process equipment.
Therefore, what is needed is a relatively simple process capable of converting non-uniform size copolymer beads into ion-exchange resins having a relatively narrow particle size distribution. Such a process could produce resins with the improved performance advantages, as previously discussed, without requiring complex and expensive equipment employed by prior methods.